1. Field of the Invention
The present invention relates to an ink jet printing apparatus and a printing method for applying ink as a printing agent to a printing medium to form an image.
2. Description of the Related Art
In these days, OA (Office Automation) equipment such as personal computers and word processors have come into wide use, and various printing apparatuses have been provided to print information outputted from this equipment on different types of printing media. An ink jet printer using ink as a printing agent is one of such printing apparatuses. In general, the printing apparatus capable of printing color images applies printing agents of three colors including cyan (C), magenta (M) and yellow (Y) or of four color including black (K) in addition to the above three colors onto a printing medium, and thereby forms an image with various colors expressed by subtractive color mixing.
However, in recent years, with a widespread use of digital cameras, there is also a need for an image quality comparable to that of silver-salt photographs even with an ink jet printing apparatus capable of easily outputting a shot image onto a printing medium such as paper. To this end, there is an ink-jet printing apparatus which is designed for enhancing image quality of print results of color-photo-like images, by printing with low-density inks of a light cyan (Lc) and a light magenta (Lm) in addition to the above ink of four colors.
Recently, single-lens reflex type digital cameras are marketed at relatively low prices, and ink jet printing apparatuses are therefore used for printing monochrome images as well as color-photo-like images. Even when printing a monochrome image, chromatic inks are used for correcting color tone, in addition to a black ink that serves as a basic tone of the monochrome image.
When forming images, print is made on the basis of a signal value that specifies an applied amount of a printing agent of each color. However, even if the applied amounts of printing agents of the respective colors are specified on the basis of these signal values to obtain desired colors, the desired colors cannot be faithfully reproduced in many cases. For example, when sizes of dots formed on a printing medium with the respective printing agents are different, even slightly, from one another, colors in a printed image composed of the collection of these dots may be observed as being slightly different from intended colors.
The above condition may occur in an ink jet printer, due to a phenomenon in which an individual difference among printing heads results in a slight difference among the amounts (volumes) of ink droplets ejected respectively from printing heads, for example. In an electro-photographic printer, the above condition may occur due to a slight difference among sizes of dots in a latent image formed on a photosensitive material. Moreover, the slight difference among dot sizes may also occur due to a relationship between a type of printing medium to be used and characteristics of printing agents such as ink and toner. Furthermore, the size of dots to be formed may also be changed due to the change of these printing apparatuses (also of the printing head in a case of the ink jet printer) over time.
In many image-forming apparatus, such a phenomenon may occur in which a color reproduced in an actual printed image is different from a color intended in a color space. In the present specification, such a phenomenon is referred to as “color deviation.”
The color deviation is noticeably observed when monochrome images, for example, achromatic images, are formed. When forming such achromatic images, only three color inks of C, M and Y or of Lc, Lm and Y are conventionally used particularly in a low-density region (a gray region).
FIG. 44 shows the content of a conventional color conversion look-up table (LUT) in a case where six colors (K, C, M, Lm and Lc) are used to express a gray line (achromatic line) of which color ranges from white to black. Here, the horizontal axis indicates a range of colors from white (W) represented by (R, G, B)=(0, 0, 0) to black (K) represented by (R, G, B)=(255, 255, 255), and the vertical axis corresponds to a density signal of each color to be outputted. As shown in FIG. 44, three color inks of Lc, Lm and Y are used to express gray in the low density region. Dots are formed discretely in a process where density is gradually increased from low to high. Consequently, ink having lower density is used to reduce granular impression that may be visually recognized when dots having relatively high reflection density scatter in a background region having relatively low reflection density. In a vicinity of an intermediate density region, outputted values for using inks of Lm and Lc approximate the respective maximum values, and it becomes difficult to express higher color density only by combining these inks. On the other hand, in this density region, granular impression due to single dots is made less recognizable since the printing medium is filled with many dots. Accordingly, by gradually adding inks of C, M and K from the vicinity of the intermediate density region, it is possible to increase density with reduced granular impression. Concurrently, outputted values for inks of Lc, Lm and Y are gradually reduced. Finally, the outputted value for ink K is set higher than values of the other inks, and thereby an achromatic image having a good tone characteristic can be expressed.
However, particularly in the low-density region, only three color inks of Lc, Lm and Y are used to express the low-density region (gray). For this reason, even a slight change in an applied amount of a printing agent of each color results in a relatively great change in hue due to an imbalance among the amounts of printing agents of these three colors. This makes it difficult to adjust the printing agents or the color materials thereof. Moreover, even a slight change in the sizes of formed dots due to a slight change in the applied amounts results in a relatively large change in colors. This means that, particularly in the gray region, a chromatic color is added to an achromatic color which is originally intended to be printed. As a result, color deviation is noticeably observed.
Japanese Patent Laid-Open No. 2005-238835 discloses a color-conversion LUT different from the above LUT.
FIG. 45 shows the content of the color-conversion LUT described in Japanese Patent Laid-Open No. 2005-238835. Ink K is used in all of the regions including from the low density region to the high density region, and the outputted value for the ink K is maintained higher than those for the other color inks. The amount of ink K increases monotonously, and the ink K, which is an achromatic color, is used in all of the density regions defined by image data in order to print an achromatic image. This makes it possible to prevent color deviation that may occur due to a slight imbalance among the amounts of inks of the respective colors in a case of expressing a monochrome image by using the chromatic inks. In other words, although the chromatic inks are also used together with ink K in the low-density region, the chromatic inks do not have a function of reducing granular impression or a function as basic colors for forming gray while balancing with each other. The outputted value of the chromatic ink merely increases monotonously even when the density changes.
An achromatic ink (gray ink, and the like.) different from ink K in density is sometimes used in place of a plurality of chromatic inks (for example, see Japanese Patent Laid-Open No. 2000-177150). That is, use of gray or black in all of the density regions can prevent color deviation in the same way as that disclosed in Japanese Patent Laid-Open No. 2005-238835.
On the other hand, use of the achromatic ink from the point in the low-density region may worsen granular impression. However, printing heads, of which amount of ejected ink per dot is sufficiently small, have recently been developed. In a case where a printing head of this kind is used, formed dots are hardly noticeable at a distance of distinct vision. For this reason, the color deviation, rather than granular impression, has an adverse effect on an image. Hence, it is effective to apply the technique disclosed in Japanese Patent Laid-Open No. 2000-177150 or No. 2005-238835.
As mentioned above, in a case where the monochrome image is formed, a large effect on the “color deviation” is demonstrated when an ink K is dominantly used from the low-density region. However, from a study on printed results of monochrome images with various densities, the present inventors have discovered that an adverse effect on the image was increased in the wide density range, due to deviation in dot-formation positions.
FIG. 46A is a schematic view showing dot arrangement in a case where an image having uniform density is printed by using an ink K dominantly. FIG. 46B is a schematic view showing dot arrangement in a case where an image having uniform density is printed by using three inks C, M and Y. Each of 46A and 46B shows a state in which dots are arranged without deviation in dot-formation positions. In other words, in both of FIGS. 46A and 47B, dots are uniformly arranged, and no granular impression occurs on the image.
Each of FIGS. 47A and 47B shows a dot arrangement in a case where the deviation in dot-formation positions occurs at the time when the same image as that in FIG. 46A or 46B is formed. As is clear from FIG. 47A, in a case where the ink K is dominantly used, the number of dots forming the image is small, that is, the coverage is low. For this reason, the amount of the ink to be applied to the printing medium is obviously less than that in a case of using the three color inks C, M and Y. Accordingly, it is apparent that the deviation in dot-formation positions is conspicuous, and that the deviation largely influences an appearance of the image, as compared with a case where the deviation in dot-formation positions occurs when the three color inks C, M and Y are used (FIG. 47B).
The deviation in dot-formation positions is caused by various factors such as: noise components including variations in nozzle shapes of the printing heads, and vibrations of the apparatus at the time of a print operation; and a distance between the printing medium and the printing head. The present inventors have recognized that one of the significant factors for the deviation in dot-formation positions was accuracy in conveying the printing medium. Normally, a roller (a conveying roller) is used as a conveyance mechanism for conveying a printing medium, and the conveying roller is rotated by the amount corresponding to a designated angle, with the printing medium pressed thereagainst. Thereby, the printing medium can be conveyed by the amount corresponding to a desired length. Accuracy in conveying a printing medium is determined by accuracy in stopping the conveying roller, eccentricity of the conveying roller, and the amount of slippage between the printing medium and the conveying roller.
The eccentricity of the conveying roller indicates a state in which the rotation axis of the conveying roller is shifted from the central axis thereof, and is a major cause of the deviation in dot-formation positions. The conveying roller is usually manufactured by controlling the amount of its eccentricity within a fixed amount. However, the stricter standard for the amount of eccentricity causes a yield reduction of the conveying roller, and thereby manufacturing costs for the printing apparatus increases. For this reason, it is not favorable that the standard for the amount of eccentricity be made too strict.
However, the eccentricity in the conveying roller causes a difference in the amount of conveyance of a printing medium even when the conveying roller rotates at the same rotation angle, and this makes it impossible to convey a desired amount of the printing medium. Specifically, when the difference is caused in the amount of conveyance, dots are formed in positions shifted from originally-intended positions along the conveyance direction of the printing medium. For this reason, dots are formed sparsely in some positions and densely in other positions. As a result, unevenness (hereinafter referred to as eccentricity-derived unevenness) occurs with a cycle corresponding to the amount of conveyance which is equivalent to one revolution of the conveying roller.
It is easy to visually recognize the eccentricity-derived unevenness particularly when a monochrome image is formed by using an ink K dominantly. In a case of such a monochrome image, since dots of the ink K are dominantly present on a white printing medium, the contrast therebetween is higher than that of dots of chromatic inks. Accordingly, in portions (white lines extending in the main scanning direction) where dots are locally sparse due to the deviation of dot-formation positions resulted from eccentric rotations of the conveying roller, color of the printing medium itself is more likely to be seen for the same reason as that explained in FIG. 47A. Thereby, a strong contrast appears between the white lines and portions (black lines extending in the main scanning direction) where dots are locally dense. As a result, this contrast is visually recognized easily as eccentricity-derived unevenness which appears periodically in the direction of conveying the printing medium.
As mentioned above, the eccentricity-derived unevenness is markedly noticeable when an achromatic monochrome image is printed by using an ink K dominantly. However, the eccentricity-derived unevenness is primarily caused by eccentricity of a conveying roller. Accordingly, even in a case of printing a monochrome image by using inks of other colors dominantly, or in a case of printing an image in which the coverage of a printing medium is low due to only a small number of ink colors to be used, the eccentricity-derived unevenness occurs with a greater or lesser degree of visibility.